Coating a fluid onto a web of material is well known. It is also known to coat a fluid onto a web in a series of discrete patches. In one system, a gravure coating process using a roll coater can be used. However, while this produces clean front and rear patch edges, the cell pattern is visible in the overall appearance, causing the patch to be optically unclear which is undesirable. Also, applying more than one type of fluid (i.e., different colors) to specific areas on a moving web requires a series of gravure coating stations with drying ovens after each coating. The repeat pattern on the gravure roll determines the location of each patch and the fluids are typically applied by a coat/dry, coat/dry, . . . , coat/dry process. The overall repeat length of a patch series is limited and set by the circumference of the gravure cylinders. Patch sizes cannot be changed except by changing the gravure cylinders.
In U.S. Pat. Nos. 3,973,961 and 4,050,410, photoconductor patches are coated onto carrier webs. A main pump provides the major supply of fluid to the die and recycle line. Excess flow is supplied to the die to obtain transversely uniform flow of fluid through the die to the web. Two dosing pumps, one upstream and the other downstream of the die, complement the main pump by adding controlled oversupply and retraction of fluid in the die for starts and stops, respectively, of the coating process. However, with this system, nonuniform light areas of coating occur on the front and back portions of the coated patch. Moreover, the coating weight increases over the front portion of the patch before decreasing toward the back portion of the patch. Also, the front and rear edges are not straight, but are convexly curved and require 2-10 mm to start and 30 mm to stop. These unacceptable variations require additional complex control equipment.
U.S. Pat. No. 4,938,994 to Choinski and a related promotional brochure entitled "Inca - 2000 Patch Coater" disclose an apparatus for patch coating a plurality of incremental printed circuit boards. During operation, the coating fluid is fed through applicator lips without continuously circulating. A single patch is coated onto a single incremental board. There is no disclosure to coat a plurality of patches on a moving web. Because the Choinski system does not coat a plurality of patches on a single board or substrate, Choinski is not concerned with coating edge sharpness because imprecise coating location does not significantly adversely affect the final product.
Moreover, a positioning piston moves the die toward and away from the board to coat and to assist in breaking the coating bead. This will not work adequately at high speeds. The Choinski system can not coat at speeds as fast as one patch per second due to the mechanical operations of moving the die and the lip seal, due to the starting and stopping of the feed pump, and due to the need for large spacing between coatings to permit cleaning and die movement. The board speed ranges from 0.30-7.62 m/min (1-25 ft/min).
Furthermore, in Choinski, the piston is inside of the die which can create shocks and cause coating defects. The piston is a flow obstruction which disrupts the coating fluid flow in the die, making a nonuniform flow distribution across the die width and leading to nonuniform coating such as streaking or banding. Also, the Choinski die is positioned perpendicular to the horizontal coating substrates. The die is oriented vertically with the die lips pointing down toward the web surface. This can lead to two problems. Air bubbles tend to accumulate in the die manifold leading to nonuniform coating and degraded patch formation due to the increased effective compressibility of the system (damping). With lower viscosity fluids, it is more likely for the coating liquid to dribble from the coater die lips onto the web between patches, requiring a lip seal. Also, as Choinski coats discrete circuit boards, there is no product beneath the die between coatings, to be ruined by dribble.
U.S. Pat. Nos. 4,729,858 and 4,831,961, to Chino et al. disclose applying a magnetic coating to a moving web. A valve helps to recirculate the coating fluid back to a reservoir when the coater stops. Recirculation occurs at the end of coating and ceases during resumption of coating after the passage of a joint between two connected webs. The valve, apparently a standard pneumatic valve, starts and stops the coater and requires about 0.5-2.0 seconds to move. Chino does not suggest coating a plurality of patches or recirculating fluid between the coating of patches.
Mcintyre, U.S. Pat. No. 3,595,204 discloses a coating apparatus that coats very thick and viscous coatings such as hot melt adhesives. This apparatus coats at thicknesses on the order of several millimeters (hundreds of mils).
U.S. Pat. Nos. 3,973,961 and 4,050,410 to Stroszynski disclose a coating apparatus which can provide relatively sharp starts of coating on the web but cannot coat sharp stops. The ends of the coatings are curved and can not be made straight. Also, the recirculation system of this apparatus recirculates fluid through the coating die, slowing the coating process.